1. Field of the Invention
The present invention relates to a nonaqueous secondary battery and a flame retardant for the battery. More particularly, the present invention relates to a nonaqueous secondary battery that has battery performance comparable to conventional batteries and that is superior in safety to conventional batteries, and a flame retardant for the battery.
2. Description of the Related Art
In recent years, reduction in size and weight of electronic devices has been remarkably progressed, and with the progress, it has been demanded that secondary batteries that are used for such electronic devices should have higher energy density. An example of secondary batteries that can meet the demand is a secondary battery including a nonaqueous electrolyte solution (hereinafter, referred to as nonaqueous secondary battery) such as a lithium-ion secondary battery.
The lithium-ion secondary battery includes a nonaqueous electrolyte solution, and the nonaqueous electrolyte solution comprises an electrolyte salt such as a lithium salt and a nonaqueous solvent. The nonaqueous solvent is desired to have high dielectric constant and high oxidation potential, and to be stable in batteries regardless of operation environment.
As such a nonaqueous solvent, aprotic solvents are used, and known examples thereof include high-permittivity solvents such as cyclic carbonates including ethylene carbonate and propylene carbonate, and cyclic carboxylate esters including γ-butyrolactone; and low-viscosity solvents such as chain carbonates including diethyl carbonate and dimethyl carbonate, and ethers including dimethoxyethane. Usually, a high-permittivity solvent and a low-viscosity solvent are used in combination.
However, the lithium-ion secondary battery including a nonaqueous electrolyte solution may suffer from leakage of the nonaqueous electrolyte solution due to a defect involving increased internal pressure caused by breakage of the battery or any other reason. The leakage of the nonaqueous electrolyte solution may lead to short-circuit between a positive electrode and a negative electrode constituting the lithium-ion secondary battery to cause generation of fire or burning. It may also lead to generation of heat in the lithium-ion secondary battery to cause vaporization and/or decomposition of the organic solvent-based nonaqueous solvent to produce gas. In some cases, the produced gas caught fire or caused rupture of the lithium-ion secondary battery. In order to solve the above-described problems, studies have been carried out to give flame retardancy by adding a flame retardant to the nonaqueous electrolyte solution.
Techniques to add a flame retardant to a nonaqueous electrolyte solution is proposed in Japanese Unexamined Patent Publication No. 2001-338682, Japanese Unexamined Patent Publication (Translation of PCT Application) No. 2001-525597 and Japanese Unexamined Patent Publication No. HEI 11 (1999)-329495, for example.
As the flame retardant, specifically, Japanese Unexamined Patent Publication No. 2001-338682 proposes phosphazene derivatives, Japanese Unexamined Patent Publication No. 2001-525597 proposes azobis(isobutyronitrile) (AIBN), and Japanese Unexamined Patent Publication No. HEI 11 (1999)-329495 proposes imidazole compounds.
While producing excellent flame retardancy, phosphazene derivatives are expected to cause unstable operation of the lithium-ion secondary battery when used with certain kinds of nonaqueous solvents or blended with a nonaqueous solvent at certain blending ratios, and when used in a certain temperature environment, in particular, at high temperature. Generally, when the lithium-ion secondary battery generates heat for some reasons, thermal decomposition reaction occurs at an interface between a negative electrode or a positive electrode and the electrolyte solution, and in the case of thermal runaway of this reaction, the lithium-ion secondary battery may be ruptured or catch fire. This phenomenon can occur even when a phosphazene derivative is blended. In addition, since the phosphazene derivative becomes a membrane on the surface of the negative electrode, battery characteristics such as cycle characteristics and environmental stability in operation may be degraded.
In an Example in Japanese Unexamined Patent Publication No. 2001-338682, a phosphazene derivative is used in a high content of 40% by volume with respect to a nonaqueous solvent. Since the phosphazene derivative has relatively high viscosity and relatively low dielectric constant, operation of a battery having a high phosphazene derivative content in a low-temperature environment causes concern about reduction in the electric conductivity of the nonaqueous electrolyte solution and degradation in the battery performance due to the reduction.
Meanwhile, AIBN is less soluble in nonaqueous solvents typified by aprotic solvents, and therefore the content thereof cannot be increased. Accordingly, AIBN may not improve flame retardancy sufficiently. Furthermore, AIBN may be electrolyzed due to charge and discharge of the lithium-ion secondary battery, causing concern about degradation in battery performance.
Likewise, imidazole compounds do not produce sufficient flame retardancy unless the content thereof is increased. However, an increased content thereof causes concern about degradation in the cycle characteristics and the environmental stability in operation.
It is therefore desired to further improve flame retardancy without degrading battery performance.